1. Field of the Invention
This invention relates generally to the manufacture of high quality glass and, more particularly, to optical quality glass used for optical lenses and glass sheet used for the production of TFT/LCD display devices that are widely used for computer displays.
2. Description of Related Art
Glass as melted from raw materials has many small bubbles of entrapped gases. These bubbles are considered defects in any glass product which requires optical properties. Bubbles of a size that can be seen by the eye or that interfere with the function of the product must be removed. The process for removing these bubbles is termed fining. Fining occurs after the glass is melted from raw materials, but before the glass is formed into a finished product. In optical quality glass this fining process is performed in a “finer” (or refiner), which is constructed of precious metal, typically platinum or a platinum alloy. The fining process is both chemical and physical. Chemicals are added to the glass such that the bubbles grow in size as they pass through the glass melting furnace and the finer. This invention is related to the physical aspect of fining, which is affected by the shape of the finer apparatus. The fining apparatus must be designed such that the removal of the bubbles from the molten glass is optimized. The finer is often very large, resulting in extremely high costs to fabricate the finer. In the fining process the bubbles rise to the top of the fining apparatus (finer) where they dissipate to the atmosphere. The size of the bubbles that are removed is a function of the size and design of the finer and the viscosity (fluidity) of the molten glass. In the glass industry these bubbles are called seeds if they are small (less than approximately 1 mm diameter) and blisters if they are large.
The prior art design which has been typically used since the start of this practice in the first half of the twentieth century is a cylindrical platinum tube with and without internal baffles. The primary innovations to date have been in the design of the baffles to alter the flow path for optimal bubble removal.
FIG. 1A shows a simplified version of a cylindrical finer (1) as known in the prior art. FIG. 2A shows a cylindrical finer with baffles. In FIG. 1A, the molten glass (2) enters finer (1) at the inlet end (3) and flows out the outlet (4). There is a free surface, or vent (5), at the outlet end (4), which is connected to the atmosphere, to allow the bubbles which accumulate at the top of the finer (1) to escape. FIG. 1B shows the typical path of bubbles (7) in a finer with a cylindrical cross-section (1), which enter the glass inlet (3) entrapped in the molten glass (2). Shown are bubbles which enter at the bottom of the finer inlet (3) and which must rise to the top of the finer at the outlet (4) so they may dissipate from the vent (5) to the atmosphere. With this cylindrically shaped finer (1) and with this size bubble the length of the finer is such that the bubbles just reach the top of the finer (1) where they are exposed to the vent (5) and can dissipate to the atmosphere.
In the baffled finer of FIG. 2A, the molten glass (2) enters the baffled finer (21) at the glass inlet end (23) and flows out the outlet (24). There is a vent (25) at the outlet end (24), which is connected to the atmosphere, to allow the bubbles which accumulate at the top of the baffled finer (21) to escape. Some of the baffles (26) have holes (22) which are sized to distribute the flow of the molten glass (2) such that the average residence time for the glass as it flows through the baffled finer (21) is more uniform. Other baffles (28) are designed to move the flow path vertically. There is often a vent (29) in front of a baffle as baffles also trap the surface bubbles into a foam-like accumulation which breaks down and dissipates into the atmosphere. FIG. 2B shows the movement of bubbles (27) through the baffled finer (21). The baffles (26) and (28) make the paths of the bubbles (27) in the baffled finer (21) quite torturous. This allows the smaller bubbles greater opportunity to coalesce together and form a larger bubble, which in turn will rise faster.